Yeti XL Bound Front Diff
 

So I've put 3 sets of packs through the XL so far, only running 4S haven't tried 6S yet. The first run I broke a front left A arm. Replaced. During the down time I shimmed the diffs for when I eventually try 6S and for good measure. Running on dirt/rock construction site area and doing some 5 foot high type jumps I came down nose forward landing the last jump as I had many before it. Then noticed trying to throttle forward the front wheels weren't moving the back wheels were. When I spun the one front wheel with my hand the other spun in the same direction. I took the diff out and found that the diff bevel gear had about 4 or 5 gears with some damage. The driveshaft bevel gear (the one I shimmed) looks OK. The diff outdrive shafts maybe are ever so slightly bent? All other gears look alright. Diff case doesn't look flexed. I did notice that the two top screws in the bulkhead holding the driveshaft bevel gear in (the one I shimmed) are ever so slightly stripped because I ever so gently continue tightening them and they never seem to get tight. Thinking maybe that on the land the force of the shaft forced forward and caused the mark on the larger bevel gear. I don't know. Axial said they'd pay to have it shipped back and fix it up send it back to me. I want to see if drive shafts and new mesh gears might fix it first (since I already ordered them yesterday). Any advices would be much appreciated. BTW I was running the rig stock except for Solar D772 servo and Castle BEC, bypassed the RX for 7.4v power, and homemade steering links using aluminum threaded tubing.

To make things a little easier let's use some more common names for the various gears of the diff. The driveshaft bevel gear is the pinion, the other bevel gear is the ring gear. The gears inside the diff cup are the spider gears.

It sounds like you're describing two separate problems. If you rotate one tire and the other tire rotates in the same direction that's an issue with the spider gears or possibly the diff outputs binding. If the diff outputs are bent they would bind on the ring gear or diff cup and cause the wheels to rotate in the same direction since the spider gears aren't rotating. Take the diff outputs out and roll them slowly across a flat surface to see if they're bent. Or simply try rotating each diff output in just the disassembled ring gear and just the dis assembled diff cup to see if there's binding.

The problem of the front wheels not turning when you apply throttle indicated a problem with the ring and pinion gears. The spider gears and diff outputs wouldn't prevent both wheels from turning, it would only prevent them from turning at different speeds. Shimming just the pinion isn't the most effective way to achieve optimal mesh but sometimes it's the only option. But when shimming the pinion it's important to make sure the pinion can move in and out of the diff case. Sometimes it's necessary to place shims on the outside of the diff case between the outer bearing and the drive cup to prevent the pinion from move in and out. After shimming did you check the mesh around the entire ring gear? Was it Notchy at all? Any play?

Thanks, I'm new hence the use of not so common terms for the gears and such. It helps to use common terms. I will need to troubleshoot with the processes you outlined tomorrow or later tonight and will report back. I will mention now that I shimmed the pinion using the instructions that yeti included (pic below). It was a serious effort for a newb like me to install the shims, but as I understand it from other more experienced folks it wasn't a walk in the park for them either. Glad to hear the Yeti's are now being delivered with the shims pre-installed. I'll also now mention that I didn't notice any notchyness in gear mesh, also didn't notice that the pinion was moving in/out of the diff cup at all. I did try turning the pinion by hand and noticed some notchyness when I turned it gripping the pinion itself, but none when I turned it gripping the drive shaft.

That's an interesting pinion. Looks like it has the cvd built into it instead of the cvd or drive cup sliding over the end of the pinion. I'm goinna have to get on axial and check it out.

This may be you just mistaking terms, but when I mentioned the pinion moving in and out, that would be in and out of the diff housing, not the diff cup. The diff cup is the part that holds the spider gears and attaches to the ring gear. When we talk about mesh we're referring to the mesh between the ring gear and the pinion. First you say it's not notchy when turning the pinion, then it is. It's usually better to rotate the pinion itself than the driveshaft when checking for smoothness in a diff. Heres how to make sure the mesh is correct. With the diff fully assembled in the vehicle, put two of your fingers of one hand on the diff outputs to hold them still. Now turn the pinion back and forth checking to see if there's any play. You want as little as possible without the diff becoming notchy. If the only way to adjust the mesh is by shimming the pinion then it's likely you'll have to compromise a little and have some play but a smooth rotating diff. Play = add more shims. Notchy = pinion is set too deep, remove shims.

Thanks I'm learning a great deal and I appreciate you taking the time to help out.

Yea it looks that the CVD is built inside the pinion itself.

Yes I meant diff housing not diff cup - the axial manual calls it the "front bulkhead", see screenshot of manual, part #AX31052. I'll call it the diff housing going forward here. Notice the three screws AXA061 which hold the pinion inside the diff housing. When I removed the pinion to install the shims I had to remove those three screws then push the pinion forward through the diff housing by gripping and pushing via the drive shaft. Those top two screws holding the pinion inside the diff housing are now loose and won't tighten. Seems the screw holes may be stripped. Seems this might be related. I will test the mesh of the pinion and ring gear soon, later tonight or tomorrow. But to be clear, in this setup at least I can't touch the pinion gear directly when the differential is fully assembled in the housing. I can turn the ring gear which then would turn the pinion gear. Is this what you meant, or would that be adequate for testing?

Ok, so the diff binding is due to the spider gears. I used the rear differential to troubleshoot. Took the rear diff out and switched components from the front to rear diff one by one to isolate the problem. Not diff outdrive shafts, ring gear, diff cup, or spider gear cross pins. It was the spider gears. I've got new spider gears on the way.

Tested the mesh between pinion and ring gear per your suggestion. Found only smooth movement and no play in the pinion depth.

Put the front diff back together with the good spider gears from the rear just to test it out, powered up, and no movement on the front wheels. Came to realize that I only needed to do some better basic observation to begin with, that the front drive shaft is not moving! Problem is not really in the front diff, pinion, ring gear at all!! So, must be something in the center transmission. Maybe it's the same problem smanders had as shown on his youtube vid with yeti xl at the track. He also had knocked out the front drive with some kind of center transmission issue. Thanks for all your help with this! I'll report back later when I find out the culprit...

And so, turns out to be a missing screw shaft in the front axle drive cup that connects to the center transmission. Wow, probly a newb kind of mistake to do all this troubleshooting to only learn it was an obvious problem. Should've known since the motor pinion came loose on the first run other screws could've been loose in that center transmission area.

Don't be too hard on yourself. That is one of the most common problems, honestly we should've caught it. Your newb terminology and the problem with the spider gears did throw us off though. At least you have a greater knowledge of your vehicle.

I'd consider replacing that bulkhead though. The stripped screws could allow the mesh of the diff gears to change and chew them up. I'm sure the bulkhead is cheaper to reace than the ring and pinion gears.

Cool thanks a lot. I definitely learned a bunch of things that will serve me well going forward. I'm definitely going to address the bulkhead I may first try to reset the threads using superglue or epoxy trick and if that doesn't work just get a new bulkhead. Thanks again for your help!

Just got the XL today and it has the shims included and needing to be installed.
Guess will pull the diffs and install the shims before any running at all .
Any other things to look for before running it ?

Also check all 4 tires before running. Tug hard on the bead and see if they come unglued. All 4 of mine needed a good re-gluing or they would have flown off the rims for sure on 6S. lol

What a PITFA the front end is to install the shims .
Rear was'nt to bad c lip being the hardest to re install .
put 5000 wt in diff .
looks like whole front needs to taken apart to get to rear of diff housing to add shims :flaming:
more like a ready to take apart than ready to run kit . Probably would have preferred a kit at this point .

Well I don't own one but everything I have read or heard about the yeti Be it the XL or the 380 or the 480 they all have the same problem/weak spot and that is there diffs. Most people have to end up replacing them after only 1 or 2 packs. The lucky ones usually can get away with 3 to 4 packs.

im at around a dozen packs, sets of packs for the xl, with both of my yetis. they are still both just fine and running great except for the caster hub i broke on the 1/10th yeti.

i didnt even shim the diffs on the xl until i ran a 6 or 8 batteries through it. you can see on the ring gear that the mesh isnt right, but there was no damage, and has been none since shimming. this thing is a beast.

I too have experienced my spyder gears causing my front diff to bind up. What I found was the 6x19x0.2mm washer behind both 20T spyder gears got con-caved/cupped like a contact lens. This pushes both 20T gears into the smaller 10T gears and I believe this is the source of the binding feeling. You can see from the pic the cupping is pretty severe. What I thought was interesting was that the spyder gears were undamaged even with the diff feeling rather gritty. Replacing those washer returned it to being butter smooth but I also ran the diff with the bowed washers. Like relay1 when I grab a front tire and spin it the other front tire spins with it (like it's locked) until you grab the other tire and really work them in opposite directions. Thou not ideal for 1/8 scale buggies and truggies I find it acts like a posi-traction and when climbing over rough terrain at slower speeds it really helps.
With tires this big and the small volume inside the diff case I really don't think you'll see much diff going from say a 20K stock weight diff fluid to 100K. You would have to use Ofnas Diff Locking Lube #10230 to really make a large diff. I'm warning you though this stuff is NASTY sticky and a super pain in the ass to clean up. lol It's even hard to get out of the container and into your diff. Here's a link to it on Towerhobbies:

http://www3.towerhobbies.com/cgi-bin...T_kaAqg28P8HAQ